Overhead projectors are used to project and enlarge an image contained on a transparent sheet onto a remote screen for easy viewing. In general, these projectors have a light source which projects light towards a fresnel lens which supports the transparency. After passing through the fresnel lens and the transparency, the light containing the image is focused and projected by a projection head assembly mounted some distance above the top surface of the fresnel lens. The head assembly includes a lens system and a mirror for changing the direction of the path of the light and for projecting the image onto the remote screen.
In traditional overhead projectors, the light source is located a predetermined distance directly beneath the fresnel lens. This design is disadvantageous in that the required spacing between the light source and the fresnel lens dictates that the projector housing be relatively large. Such constructions are further disadvantageous in that cooling becomes significant when the light source is mounted in the projector body directly beneath the fresnel lens and transparency. One solution to the problems associated with traditional overhead projectors was the introduction of folded optics into the optical system. In a folded optics system, the light source is mounted outside of the optical field of the fresnel lens. The light is directed toward a mirror which is mounted beneath the fresnel lens. The mirror redirects the light towards the fresnel lens carrying the transparency. The mirror is mounted at an angle relative to the fresnel lens and the light source such that it appears as if the light source is mounted directly beneath the fresnel lens as in a traditional projector. The term "folded" is used to describe this type of optics because the light is "folded" when it strikes the mirror in order to direct the light toward the fresnel lens. An overhead projector incorporating folded optics can be constructed with a reduced height and significantly simplified cooling mechanisms relative to a projector without such folded optics. However, even folded optics projectors have an inherent height limitation. That is, sufficient spacing is required to position the mirror properly so it will reflect the light from the light source, through the lens to the head assembly. Thus, although folded optics systems do present an improvement, such machines still prove to be bulky and unwieldy to easily handle and manipulate.